lynnb
Veteran
Roger Cicala at LensRentals has published an update to his investigations into autofocus. Although specifically mentioning Canon, the article mentions principles that would affect most if not all designers.
Briefly:
- Contrast-detect is more accurate than phase-detect
- Contrast-detect is "about as accurate as the most careful manual focusing"
- improvements in autofocus accuracy depend as much on improvements in lens drive stepper motor accuracy as on camera hardware
- introduction of closed-loop AF systems in ultrasonic-driven lenses using rotation detectors has increased focus accuracy
- the further apart the pairs of high precision (cross-type) AF sensors, the more accurate the focus
What this means:
- improvements in both camera body AF systems and lens stepper motor accuracy work together to improve overall AF accuracy
- more accurate camera AF systems demand updated lenses with more accurate AF stepper motors, i.e. if you upgrade a digital camera body you may need to upgrade all your lenses to take advantage of the camera's better AF capability (if you rely on, or need to use AF)
- existing AF lenses may become obsolete with each new generation of improved AF camera bodies
Seems to me the cost of periodic rangefinder adjustments in digital M bodies using manual focus lenses, is a lot less than the upgrade cycle of digital autofocus systems. As sensor resolution increases focus becomes more critical. It's reasonable to assume future mirrorless models with 40+ megapixel sensors (X-Pro2?) may require a complete lens set upgrade in order to guarantee focus accuracy if you rely on AF. As Roger Cicala shows, this is already happening with Canon - the 5DIII's improved AF is only effective when paired with newer lenses with improved steppers.
Briefly:
- Contrast-detect is more accurate than phase-detect
- Contrast-detect is "about as accurate as the most careful manual focusing"
- improvements in autofocus accuracy depend as much on improvements in lens drive stepper motor accuracy as on camera hardware
- introduction of closed-loop AF systems in ultrasonic-driven lenses using rotation detectors has increased focus accuracy
- the further apart the pairs of high precision (cross-type) AF sensors, the more accurate the focus
What this means:
- improvements in both camera body AF systems and lens stepper motor accuracy work together to improve overall AF accuracy
- more accurate camera AF systems demand updated lenses with more accurate AF stepper motors, i.e. if you upgrade a digital camera body you may need to upgrade all your lenses to take advantage of the camera's better AF capability (if you rely on, or need to use AF)
- existing AF lenses may become obsolete with each new generation of improved AF camera bodies
Seems to me the cost of periodic rangefinder adjustments in digital M bodies using manual focus lenses, is a lot less than the upgrade cycle of digital autofocus systems. As sensor resolution increases focus becomes more critical. It's reasonable to assume future mirrorless models with 40+ megapixel sensors (X-Pro2?) may require a complete lens set upgrade in order to guarantee focus accuracy if you rely on AF. As Roger Cicala shows, this is already happening with Canon - the 5DIII's improved AF is only effective when paired with newer lenses with improved steppers.
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ColSebastianMoran
( IRL Richard Karash )
Thanks, Lynn, for posting this. Thanks also for your excellent summary.
This is very interesting stuff. And, it's from actual testing, not theorizing.
An overall conclusion: Phase Detection AF (the kind we usually use) can be as accurate as the best focusing technique, but only with the newest lenses and the newest pro bodies.
He's tested only Canon so far, but promises to do the same with Nikon next.
This is very interesting stuff. And, it's from actual testing, not theorizing.
An overall conclusion: Phase Detection AF (the kind we usually use) can be as accurate as the best focusing technique, but only with the newest lenses and the newest pro bodies.
He's tested only Canon so far, but promises to do the same with Nikon next.
giellaleafapmu
Well-known
I wonder what the final target would be, autofocus is already damn good, Sony's A77 thanks to its translucent mirror technology can already keep focussing while shooting at 12f/s with a camera/lens combo which costs less than 2000US$ and include a 24-75mm equivalent zoom with constant 2.8 aperture and I believe Nikon and Canon have similar performance althought in higher end bodies. Nikon's 1 shows that it is not far away the time in which something similar will be available in "mirrorles" cameras (I know this is not the best way to describe them in this forum but I am using the common term). The new full-frame from Sony apparently will have some 102 (!!!) cross-type autofocus points. What are ingeneers aiming at? Continuous focus at 100f/s? A zirillion autofocus points. That would be maybe useful in scientific photography but I can hardly immagine any other situation in which one would not be fine with what is already available. Oh, ok, I know when that will come out I will probably start convincing myself that actually do even though I seldomly shot anything which moves... 
GLF
GLF
lynnb
Veteran
Accurate autofocus is dependent on sensor resolution (other factors also are at play). An AF system designed to accurately focus on an 18-24 Mp sensor may be substandard on a 40+ Mp sensor. That is the real problem. As sensors improve, people upgrade camera bodies (who regularly uses a 4-6Mp digital camera nowadays?). Unfortunately, they may also have to upgrade lenses. The stepper motors in current lenses may not be able to take advantage of the improved AF systems - systems that are necessary to make sure the increased resolution of higher pixel density sensors is captured. This is the hidden cost of technological advancement.
The number of cross type AF sensors is only part of the AF story. What this article is saying is that lenses need to be of the same generation as the sensor/AF system, in order for AF to work to the required accuracy.
giellaleafapmu
Well-known
Yep,
maybe the right question would be: who print larger than 8x10" or at more than 300dpi subject which require fast autofocus? I am sure that you can measure and convince yourself that each two years you need to get four times more megapixels and that as a consequence you need to get each time a completely new set of lenses to "get out the most of your autofocus" but I doubt your clients will see that in your pictures.
GLF